The present invention relates to a method of vacuum forming from thermoplastic sheets and a mold therefor.
A method is already known which forms products from thermoplastic resin sheets by vacuum forming. One example of the conventional vacuum forming method and mold therefor is illustrated in FIG. 1. The vacuum forming machine shown includes a bed 1 on which a mold 3 is mounted with gasket 2 interposed therebetween. The mold is hollow and has a plurality of vacuum suction holes 4 and 5 distributed over the entire mold or partially where necessary as in the top and bottom thereof. A sheet 7 is tightly held in place by clamping frame 6 and is softened under heat. Under this condition, the mold 3 is thrust against the sheet 7 from below to the position shown. After the mold 3 has moved into sealing engagement with the sheet 7, the air in the mold 3 is exhausted by a suction pipe 8. Thereupon, a vacuum is established in the space between the sheet 7 and the mold 3 whereby the sheet 7 is pressed into contact with the outer surface of the mold by the atmospheric pressure and assumes a shape shown by reference numeral 7'. In the process described, the atomospheric pressure of approximately 1 kg. per cm.sup.2 is applied to the surface of the mold 3, so that a large sized mold having a surface area of 4 m.sup.2 will be subjected to a load of approximately 40 ton. The mold is usually made as an alminium casting, which must have a thickness on the order of 30 mm in order to avoid deformation under such load, and which must be additionally reinforced with ribs, posts or the like located on its interior surface. Where the mold is made from resin material or the like, a packing 9 of material such as gypsum, cement or the like must be provided to fill its interior, as shown in FIG. 2, in order to prevent the mold from collapsing under the atmospheric pressure upon forming. All these add to the weight and cost of the mold as well as the inconveniences in its handling. Further, the mold lift mechanism of the vacuum forming machine must be capable of lifting the heavy mold which weighs in excess of 1 ton.